1. Field of the Invention
The present invention relates to a speech coding apparatus, and more particularly to a speech coding apparatus having a VOX (Voice Operated Transmitter) function.
2. Description of the Related Art
Conventionally, a speech coding apparatus of the type which has a VOX function is used to stop, when input voice is silent, transmission on the coding side and produce a certain kind of background noise on the decoding side as disclosed, for example, in Japanese Patent Laid-Open Application No. Heisei 5-122165 which is directed to a speech signal transmission method.
FIG. 7 shows in block diagram a general construction of a conventional speech coding apparatus. Referring to FIG. 7, the speech coding apparatus shown includes an input terminal 1 for a speech signal, a voice presence/absence discrimination section 2, a high efficiency coding section 3, a unique word production section 4, a data switching section 5 and an output terminal 6.
In a digital radio transmission system, a speech signal inputted from the input terminal 1 is cut out and processed for each frame. The length of the frame is, for example, 40 ms.
The voice presence/absence discrimination section 2 receives a speech signal for one frame from the input terminal 1 as an input thereto and discriminates whether or not the current frame is a voice present period in which voice is present or a voice absent period in which voice is absent. The high efficiency coding section 3 receives a speech signal for one frame from the input terminal 1 as an input thereto and converts the speech signal into high efficiency codes. The unique word production section 4 produces a preamble signal and a postamble signal. The preamble signal is used to indicate, upon transition from a voice absent period to a voice present period, the transition to a speech decoding apparatus (not shown). The postamble signal is used to indicate transition from a voice present period to a voice absent period and indicate that background noise updating codes are to be transmitted in a next frame. Further, the postamble signal is transmitted after every (T+2) frames while a voice absent period continues. It is to be noted that both of the preamble signal and the postamble signal have signal patterns which are not present in a high efficiency code system in an ordinary case. The data switching section 5 selects one of a high efficiency signal outputted from the high efficiency coding section 3 and a preamble signal or a postamble signal outputted from the unique word production section 4 in accordance with a result of discrimination of the voice presence/absence discrimination section 2 and outputs a selected one of the signals through the output terminal 6. The output terminal 6 transmits data selected by the data switching section 5 to the speech decoding apparatus.
If it is discriminated by the voice presence/absence discrimination section 2 that a current frame is a voice present period, then the data switching section 5 selects a high efficiency code produced by the high efficiency coding section 3 and outputs it through the output terminal 6. On the other hand, if it is discriminated that the current frame is a voice absent period, then the coding apparatus performs a VOX process which have such steps as described below:
(1) The data switching section 5 is switched so that a postamble signal produced by the unique word production section 4 is outputted through the output terminal 6. PA0 (2) The, the data switching section 5 is switched so that a high efficiency code produced by the high efficiency coding section 3 is outputted through the output terminal 6. It is to be noted that a high efficiency code transmitted next to a postamble signal is hereinafter referred to as background noise updating code. PA0 (3) Thereafter, the output through the output terminal 6 is stopped for a fixed time. It is assumed that, in the following expression, the fixed time is T frames (T is a constant). PA0 (4) After the fixed time (T frames), the processes beginning with (1) above are repeated.
However, also during a voice absent period, the voice presence/absence discrimination section 2 performs voice presence/absence discrimination for each frame. If presence of voice is detected during a speech absent period, then in the frame, a preamble signal is produced by the unique word production section 4 irrespective of the VOX process. The data switching section 5 selects the preamble signal produced by the unique word production section 4 and outputs it through the output terminal 6. Then, ordinary processing in a speech present period is performed beginning with the following frame. In particular, the data switching section 5 selects a high efficiency code produced by the high efficiency coding section 3 and outputs it through the output terminal 6.
The speech decoding apparatus receives a coded signal transmitted from the output terminal 6 of the speech coding apparatus. When a postamble signal is received, the speech decoding apparatus recognizes that the current frame is a speech absent period, and produces, for a period of T frames, background noise using a background noise updating code received in a frame next to the postamble signal. It is to be noted that background noise is updated each time a new background noise updating code is received. If a preamble signal is received during a speech absent period, then the speech decoding apparatus recognizes that a speech present period begins with the next frame, and produces decoded voice from received high frequency codes.
In the following description, a frame with which a postamble signal is transmitted is referred to as postamble signal transmission frame; a frame with which a background noise updating signal is transmitted is referred to as background noise updating frame; a frame with which transmission is stopped is referred to as transmission stop frame; a frame with which a preamble signal is transmitted is referred to as preamble signal transmission frame; and any other frame than the frames mentioned is referred to as ordinary transmission frame.
The prior art described above has a problem in that background noise produced by the speech decoding apparatus in a voice absent period is an unnatural sound.
The first reason is that, since the background noise updating code outputted from the speech coding apparatus is transmitted after every (T+2) frames ((postamble signal transmission frame)+(background noise updating frame)+T frames), background noise produced from a same background noise updating code continues for (T+2) frames.
The second reason is that, since background noise is updated immediately after a background noise updating code is received, if the variation of the power value of background noise across the updating is large, then the background noise gives, at a break of the background noise (at the time of updating), an unfamiliar feeling to a listener decoded speech of the speech decoding apparatus.